1. Field of the Invention
The present invention relates to a process and device for determining the mean value of the current delivered by a controllable valve rectifier bridge.
It applies more especially but not exclusively to the servo-control of an electronic speed variator for a DC motor using a controllable valve rectifier circuit, for example a thyristor power bridge, of the Graetz circuit type fed with a multi-phase current.
2. Description of the Prior Art
It is known that in a three phase current variator of this kind, the rectifier circuit is usually formed of a bridge comprising three thyristor pairs connected in the following way:
the two thyristors of the same pair are connected in series in the same direction, PA1 the junction point between the two thyristors of the same pair is connected to a corresponding phase of the three phase network, PA1 the two free ends of the thyristors of the same pair are connected respectively to the two supply terminals of the motor. PA1 successive integration, by means of a synchronous integrator, of a signal representative of the current supplied by the rectifier, each integration cycle having a duration equal to the period of the synchronization signal but being possibly phase shifted with respect thereto ; PA1 sampling, at the end of each integration cycle, of the integrated signal obtained ; PA1 temporary storage of the sampled signal (blocking) ; resetting of the synchronous integrator after each sampling ; PA1 the analog-digital conversion of each of the sampled signals stored. PA1 the measurement, on the alternating side, of the difference between the currents flowing in two of the supply phases, PA1 determination of the absolute value of this difference, and, PA1 attribution to the absolute value of the resulting signal of a coefficient k so as to obtain an output signal which forms a continuous image of the current delivered by the rectifier bridge.
This circuit should further comprise a means for successively switching on the thyristor pairs of the bridge in synchronism with the zero cross-overs of the compound voltages of the mains, but with a certain phase-shift .phi. with respect to the rectifier positive cross-over points and, consequently, detectable with respect to the zero cross-over times of the phase voltages of the network (the rectifier positive cross-over point corresponding to the point of positive equality, at the same time, of two successive phase voltages of the network). Adjustment of the output voltage of the rectifier is then obtained by varying this phase shift .phi..
Furthermore, the current delivered at the output of the thyristor rectifier is formed by a succession of curves passing from one to the other in synchronism with the zero cross-over of the phase voltages of the network but with said phase shift .phi.. This is why, in numerous applications, it is not the instantaneous value of this current which is used, which is not significant because of its variations, but the mean value of this current.
According to the technique used at present, this mean value of the current is obtained by appropriate filtering. However, it has proved that this process is not suitable in the case of regulating a digital electronic speed variator because of the considerable delay which it introduces into the servo-control feedback loop.
The invention provides then a process for determining the mean value of the current delivered by a thyristor rectifier bridge, which does not comprise this drawback.